1. Field of the Invention
The object of the invention is an improved method and arrangement for permitting and performing cell reselection made by a terminal of a cellular radio system. The invention relates particularly to permitting and performing cell reselection in situations where the terminal requires a special service, such as GPRS (General Packet Radio Service).
2. Brief Description of Related Developments
In order to understand the background of the invention we first describe as examples prior art solutions for changing the active cell in the GSM system (Global System for Mobile telecommunications) and in the associated GPRS data packet transmission service. FIG. 1 shows the basic structure of a GSM network. In a mobile communications network there is at least one core network (CN) and one or more radio access networks (RAN). The core networks are formed by various exchange systems which in addition to versatile data transmission facilities can provide various value-added services. A core network comprises switching centres MSC (Mobile services Switching Centre), other network elements which in the GSM system are for instance the nodes associated with packet radio communication, the SGSN (Serving GPRS Support Node) and the GGSN (Gateway GPRS Support Node) and the associated transmission systems. The radio access networks are located between a core network and the terminals. A radio access network comprises base stations (BS) and a radio network controller (RNC). Each base station has a permanent connection to the respective radio access network controller. On the other hand the controllers of different radio access networks have a permanent connection to at least one node of the core network. One or more radio access networks can operate between the terminals and the core networks, and via a certain radio access network a terminal can communicate with several core networks.
A terminal of a cellular radio system always tries to select a certain base station and operate (camp) within its coverage area, or cell. Traditionally cell reselection was based on the measurement of strength of the received radio signal, either at the base station or at the terminal. For instance, in the GSM system each base station transmits a signal on the so called beacon frequency, which is different for neighboring base stations. In the GSM system a base station transmits on the so called BCCH channel (Broadcast Control Channel) the parameters p1 and p2 to the terminal, whereby these parameters are used to calculate of the so called C values. For instance the C1 value commonly used in the cellular GSM network is calculated from the equation below:
C1:=(A-Max.(B,0)), where
A:=average level of the received signalxe2x88x92p1 (dB)
B:=p2xe2x88x92maximum transmission power of the terminal (dB)
p1:=allowed minimum value for the received signal
p2:=allowed maximum transmission power for a terminal.
In addition the terminals must measure the reception levels of the broadcast signals which they have received, so that they will be able to calculate the C1 value of each received cell. The cell which has the highest calculated C1 value is the most advantageous regarding the radio connection. In order to optimise cell reselection the network can also transmit additional parameters which enable the use of so called C2 values. A more detailed description is presented in the reference [11]. The base stations transmit to the terminals information about the BCCH frequencies used by the neighboring cells, so that the terminals will know on which frequencies they must listen in order to find the BCCH transmissions of the neighboring cells. In each cell the BCCH channel transmission also contains information about how the terminals can request random access in the respective cell in order to establish a connection.
The cells of the GSM system offer to its user the basic services: voice transmission, low-speed data transmission, and various short message services. The operation of the GSM system is standardised by ETSI (European Telecommunication Standardization Institute). The cells in the GSM system are interleaved regarding their coverage areas, so that at the cell borders it is necessary to decide which cell the terminal in question shall use. In non-active mode the terminal decides which cell to camp on, but in active mode the decision is influenced, in addition to the quality of the connection, also by other factors, for instance by the loading degrees of the cells of the network. In the dedicated mode the core network decides which of the two cells the terminal shall use, and the timing of the cell change, which it informs to the terminal. According to the ETSI standard, in the dedicated mode terminal can not independently access the service of another cell. In the idle mode the terminal decides, on the basis of the C1/C2 values which it has calculated for a number of neighbor cells, to which cell it shall belong. The actual timing of the cell change either in the dedicated mode or in the idle mode is not very important regarding the terminal, because all cells of the system normally provide the same basic services, and the user of the terminal will not notice the change from one cell to another.
However, new services are being constructed on the platform provided by the GSM system, so that these new services are able to provide various value-added services to the user of a terminals. These services might not be provided over the whole coverage area of the GSM network, but only in some limited areas. One such service could be GPRS. In the GPRS service area the user of a terminal can choose whether he uses the basic GSM services or whether he utilises the value-added services provided by GPRS. FIG. 2 shows the provision of a GPRS service over a limited area. Of the cells in FIG. 2 the cells 1, 3, 6 and 8 offer the GPRS service in their areas, but in all other cells only basic GSM services are available. The figure also shows the path of a mobile terminal from point P1 to point P7. In the active mode at the cell border regions the cell change may be controlled by the network, or it might be controlled by the terminal. Thus the following situation for a terminal could occur: the terminal obtains GPRS services on the distances P1xe2x86x92P2, P3xe2x86x92P4 and P6xe2x86x92P7. On the distances P2xe2x86x92P3, and P4xe2x86x92P5xe2x86x92P6 only the basic GSM services are available. If the user started in P1 with an active GPRS connection, he will in the worst case notice that the GPRS services are interrupted for a shorter or longer period of time during an active session when moving towards P7. The user will be surprised by the service interruptions, because in the user""s opinion he is moving within the GPRS coverage area made known by the service operator. Another example of an undesirable case is that the terminal can not establish a desired GPRS connection to a cell supporting the GPRS service when the terminal is located in an unsuitable place, for instance in FIG. 2, on the distance P2xe2x86x92P3. This situation could occur even when the terminal is located clearly within the GPRS coverage area defined by the network service, if the terminal, due to C1/C2 values are forced to reselect to a cell not supporting GPRS service. A third inconvenient case is the point P7, which is located at the border of the cells 6 and 7. In the worst case the terminal has to make repeated cell changes between a cell supporting GPRS service and a cell not supporting GPRS service. This means that the user will experience the situation where he at certain times has GPRS service available and at other times does not have GPRS service available, even though he is at the location all the time.
The GPRS standards and the proposed revisions to them have tried to solve the above presented problem in the following way. The network permits that all terminals supporting GPRS and camping in a GPRS cell can favour certain cells at the expense of other cells. For this purpose the network can use C31/C32 values, which are described in more detail in the reference [11]. Thus a terminal stays as a user of services of a certain cell, even when the received power from a beacon frequency of a neighboring cell is higher than that of the current serving cell according to the terminal""s measurements, provided that the C-values of the current cell fulfills certain criteria and that the serving cell has enough transmission capacity on hand.
In the example of FIG. 2 the cells 1, 3, 6 and 8 could be such favourable cells. The parameters enabling operation of this kind are called offset parameters. In basic GSM the different cells transmit their offset parameters on their beacon frequency. The offset parameters for each neighboring cell might have different values. The terminal reads these parameters for a certain selection of neighbor cells so that the terminal can calculate the C values for each of these cells. In a GPRS supporting cell the terminal might receive all necessary offset parameters, also those for neighboring cells, from the current serving cells broadcast information, so that the terminal in this case can calculate the necessary C-values for selected neighbors, without reading the neighbors broadcast information on their beacon frequencies. In either way the terminal can use the calculated C-values to determine if it can remain a user of its current cell, even though the signal level of a neighboring cell according to the power level measurements is higher than the signal of the current cell. This is true so as long as the C value of another cell does not exceed the C value of the cell in question.
A prior art network transmits said offset parameters in the system information messages on the BCCH channels transmitted by the base stations of the cells; in the basic GSM system these messages are called SI messages and in the GPRS services they are called PSI messages (Packet System Information message). However, the level of the received signal varies in an unpredictable way due to the characteristics of the transmission path. Thus the result from the calculation of the C values can also vary in an unpredictable way from one location to the next or just in time, and it is therefore difficult to obtain a 100% certainty about the complete or precise coverage area of each cell. From time to time this can cause undesired areas, a kind of holes, in a planned continuous GPRS coverage area of a number of cells, whereby said service may be surprisingly missing. Thus, only by favouring cells in the case of FIG. 2 it is not possible to guarantee with a 100% certainty that GPRS service will be available from point P1 to point P7xe2x80x94or maybe even within the area covered by cells 1,3,6 and 8.
Because the service levels of the neighbor cells are not checked in advance when operating in a prior art GSM network, then due to a cell reselection the terminal may enter a cell which does not offer the GPRS service required by the terminal. The example case in FIG. 3 presents the operation according to the specifications when a terminal performs a cell change. In the initial situation, step 30, the terminal can be either in the idle state or in the active state. In step 31 the terminal examines the need to change the serving cell. The best of six neighbor cells in the terminal""s list of neighbor cells is selected, if a need to change the cell is detected, step 32. However, before the selection there is no check about which services the selected cell supports. In step 33 the terminal performs configuration to the new cell, whereby the terminal shifts to operate on the frequencies of the new cell and obeys the timing information given by the cell. In step 34 the terminal receives the system information SI or PSI of the new cell. In step 35 it is examined whether sufficiently information has been obtained from the new cell. If not, there is a return to step 34. If all required information has been received, then there is a move to step 36, where a test is made whether the selected cell supports the GPRS service. If the cell supports GPRS there is a return to step 30 and the operation the terminal may continue its GPRS service as normally. If the selected cell does not support the GPRS service there is a move to step 37, where the desired service is not available to the terminal and therefore the terminal may not continue its GPRS service. Thus the prior art operation supports only the provision of cell based GPRS services, even though the user rather would require a regional GPRS service.
The object of the presented solution is to present a method and an arrangement for avoiding the disadvantages of prior art. An essential idea of the invention is that the network knows the service level required by a particular terminal as well as the service level and loading situation of the terminal""s current cell and its neighbor cells. On the basis of this information the network transmits to the terminal via the base station serving the terminal, in a so called system information message, information about whether the terminal is allowed to perform a special cell access request when it desires. This request from the terminal informs the serving base station about its desire, to utilize a specific service of its current cell or a neighbor cell. In this way the network might provide the service in the current cell or allow or command the terminal to change serving cell, so that the terminal in question would get the service required by it. This information is transmitted to the terminal most preferably in the signalling message of a certain service, to which certain service the terminal is connected. In the same connection or message the terminal is also given information about whether it can favour cells providing GPRS service, at the expense of the basic GSM cells. Favouring can mean that the terminal remains a user of the current cell or changes to be a user of a new cell (cell reselection) supporting GPRS.
For instance, when the terminal operates in a cell supporting the GPRS service, then the information about whether the terminal when required is allowed to transmit a special cell access request to the network is transmitted in the PSI message of the packet channel of the GPRS system. In the simplest case the invention can be realised in the GPRS system by adding information elements to the PSI signalling message of the packet channel, whereby these added elements tell whether the terminal can transmit, when required, a special cell access request to its current cell. Also it might be indicated in the serving system information whether the use of the special access request may is allowed in a neighboring cell if/when this neighboring cell becomed the serving cell. If the terminal does not operate in a special service, such as in the GPRS service, then the information can be transmitted in the system information message of the basic system, such as in the SI message.
Thanks to the invention a terminal operating in a certain service can remain a user of its current cell, or directly change the current serving cell to another cell providing the corresponding service, without making intermediate cell reselections to cells possibly not supporting the required service. Thus for instance a terminal operating in the GPRS service does not have to switch-over to receive the system information messages on the basic GSM channels transmitted by neighbor base stations in connection with cell reselection, but the terminal can directly access, on the GPRS level, a new cell providing the GPRS service.
The operation of the terminal in order to find a new cell can be directed more efficiently than in the prior art if the terminal gets a permission according to the invention to transmit, when required, a special cell access request to its cell, and when it has got said permission, then it gets information from the network about which of the neighbor cells are available to the terminal for cell change. In an arrangement according to the invention the terminal transmits to its current base station, associated with a special cell access request, the identification and level information which it has measured concerning its neighbor cells. According to the needs the terminal maintains different lists of such neighboring cells, which it may access in the near future.
A method according to the invention for controlling the cell reselection of a terminal is characterised in that the terminal is authorised to perform a special cell access, and that the for the cell reselection required by the special cell access the terminal transmits to the base station an additional message which comprises the results of the neighbor cell measurements made by the terminal.
A base station according to the invention in a cellular radio system is characterised in that
it comprises means for processing a special cell access request,
it comprises information about which base stations are available to the terminal so that it can move the terminal to be a user/remain a user of one of these base station, and
it is arranged to include said information and that the terminal is permitted both to request a special cell access and to perform a cell reselection, into the signalling messages addressed to the terminal.
A terminal according to the invention in a cellular radio system is characterised in that it is provided with
means for making a special cell access request to the base station of the current cell,
means for receiving and interpreting cell reselection information from the signalling messages transmitted by the base station, and
means for performing said cell reselection on the basis of said information.
The invention further relates to a cellular radio system which is characterised in that it comprises information available to the base station, which information concerns a number of neighbor cells adjacent to the cell of said base station, and which information contains information about the service level provided to the terminals by the neighbor cells, whereby said system is arranged, when a special cell access request has been received, to transmit said information in a signalling message from the base station to the terminal for selecting a new serving cell.
Some advantageous embodiments of the invention are presented in the dependent claims.
An advantage of the invention is that the network can, when required, give a single terminal in advance the authorisation to stay as a user of its current cell or to move to be a user of a cell which offers GPRS services. In doing so it is possible to provide value-added services regionally, instead of the provision of a service based on individual cells. For the user this means that inconvenient service breaks will decrease substantially when the invention is used.
According to the invention a particular terminal is able to transmit a special cell access request to its current cell and, when required, to make a cell reselection according to the standard when the terminal is in the so called idle state, in which there is no active data transmission between the terminal and the base station, or according to the invention it can also be made during active data transmission. This could make the cell reselection faster and also make the cell reselection more flexible compared to prior art method.
The invention also furthers the focusing of the cell reselection so that the loading on the terminal and on the radio signalling caused by measurements and message reception regarding any new cells is as low as possible.
A further advantage of the invention is that when the resources of a certain cell are exhausted, then another cell can be allocated to the terminal, and thus a broader service can be offered to the terminal.